    \begin{tikzpicture}
    %------------------------------------------------------------------------------------
    %    图片2.7
        \node (S) at (0,0) {S}
        [level 1/.style={level distance=3em, sibling distance=2.5em},
        level 2/.style={level distance=3em, sibling distance=2.5em}]
            child {node (1a) {a}}
            child {node {A}
                child {node {A}}
                child {node (2b) {b}}
            }
            child {node {c}}
            child {node {B}
                child {node {b}}
            }
            child {node (1e) {e}};
    \end{tikzpicture}